1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) radio communication system for mobile objects, in particular, a system suitable for high-speed mobile objects, such as a vehicle.
2. Description of the Related Art
Heretofore, digital mobile communication systems have been used for telephones and data transmission. In such conventional mobile communication systems, a TDMA (Time Division Multiple Access) system or an FDMA (Frequency Division Multiple Access) system is used. Thus, multiple access is realized by physically dividing a time period and a frequency.
Lately, the CDMA system has been proposed and its development is in progress. In the CDMA system, a plurality of mobile stations simultaneously use the same frequency band. Further, multiple access is realized by using codes based on spread spectrum communication technology. For example, in DS (direct sequence) type spectrum diffusion, the diffusion modulation and demodulation of communication signals is performed using PN (pseudorandom noise) codes. By allocating a different PN code to each user, it is possible for a plurality of users to perform their respective communication using frequencies in the same band.
Adoption of the CDMA system can lead to sharp improvement in frequency efficiency compared with the conventional systems. In other words, the number of mobile stations capable of being simultaneously connected with the base stations can be increased. Therefore, the CDMA system is expected to be a system for coping with the increase of subscribers which is a type of problem in mobile communication systems. Moreover, the CDMA system has come to public notice due to its advantages that it has high confidentiality, soft hand-off between base stations is possible, and the like.
In the CDMA communication, a communication speed capable of securing stable communication is limited depending on the traveling speed of a mobile object. Therefore, when a mobile object which travels at a high speed, such as a vehicle, was provided with a mobile station, a communication speed had to be set at a sufficiently low value. This made it difficult to meet the needs for high speed communication. This problem will be subsequently explained exemplifying a vehicle as a mobile object.
In the CDMA system like other communication technologies, it is desired to make the communication speed as high as possible. The CDMA system is applied to mobile computing and data transmission is carried out between a terminal device on the mobile station side and a base station. Due to simultaneous data transmission of dynamic images, static images, voice, and the like, an amount of data to be transmitted tends to be increasing. Thus, it is required to perform high speed and accurate transmission for the following reasons: there is a need to reduce communication expenses by shortening a data transmission time; in the case of mobile objects, it may be limited in a time period which is possible to maintain continuous connection.
As is the case with a portable terminal for pedestrians, if it is assumed that communication would be performed in a stationary state or a state of traveling at a low speed, the communication speed of the CDMA system can be set at a considerably high speed. Even in the case of a vehicle, if it is at a standstill or traveling at a low speed, similar high speed communication can be performed. However, if the traveling speed is accelerated during high speed communication, the communication cannot be normally performed. This is because, as will be explained later, Doppler shift exerts more influence on the communication, for example. Therefore, if it is assumed that the terminal would be used in a vehicle, the communication speed will have to be set at a low speed.
For reference, in the conventional systems using the TDMA and FDMA systems, it is planned to transmit data at a high speed by simultaneously allocating a plurality of data channels to each user. For example, in the case of PDC (Personal Digital Cellular) in Japan, a transmission speed of 9.6 Kbps is accelerated to 28.8 Kbps in digital mobile communication.
Further, also in the conventional systems, a maximum data transmission speed can be obtained in mobile computing performed on the assumption that data is transmitted to a portable terminal carried by an ordinary pedestrian or a portable terminal in a stationary state. However, if the traveling speed of a terminal is 10 Km/h or more during communication, as is the case with vehicles, it will be difficult to realize a transmission speed similar to the speed at the time of standstill or traveling at a low speed. Therefore, this will cause such a problem that it is difficult to meet the need of restricting the transmission time.
Japanese Patent Laid-Open Publication No. Hei 8-149543 relates to a PHS (Personal Handyphone System). As is well known, in the PHS, the TDMA or FDMA system is used. According to the Publication mentioned above, a traveling speed of a mobile object is detected based on an input signal transmitted from a GPS (Global Positioning System) satellite. When the traveling speed reaches a certain value or more, an alarm is given to a user of the mobile terminal to inform that it will be difficult to perform communication. In this case, the user cannot continue the communication without decreasing the traveling speed. In other words, the user cannot continue the communication while traveling at a high speed.
The subject is now returned to the CDMA communication system for mobile objects. In the CDMA system, the communication speed is restricted depending on the traveling speed of a mobile object as described above. Specially in a wide band CDMA (W-CDMA or Wide-CDMA) system, between a terminal device for pedestrians used only at a low traveling speed and a terminal device for mobile objects, such as a vehicle, there is a big difference in communication speed which can be set. The communication speed of the former is several times as high as that of the latter. Therefore, there is a complete difference in environment of mobile computing during the CDMA communication between these two types of terminal devices. In particular, as is the case with ITS (Intelligent Transport Systems) technology, data communication in vehicles tends to be becoming more and more important. Thus, it is desired to make a data communication speed as high as possible.
The Doppler shift is one of the reasons why the communication speed is restricted depending on the traveling speed. When the traveling speed is high, the Doppler shift exerts an influence on inverse diffusion in diffusion demodulation. In particular, if the traveling speed is not constant and always changes, like the case with vehicles, it will be difficult to follow the Doppler shift as will be described hereunder.
If the DS system is adopted, as described above, the diffusion modulation and the demodulation is performed using PN codes. In the case of vehicles, the PN codes are rendered correlative by a diffusion demodulation process (inverse diffusion). The PN codes have waveforms in which normally pseudo +1 or xe2x88x921 appears at random, as shown in FIG. 8. A waveform corresponding to one +1 or xe2x88x921 is referred to as a chip. A time period of maintaining one chip is referred to as a chip time. The number of chips per unit time is referred to as a chip rate. The chip rate and the bandwidth of communication signal correspond to each other. Thus, if the chip rate becomes great, the bandwidth will become wide.
Here, with the increase of the communication speed, the chip rate and the bandwidth become great. For example, the communication speed is faster when the chip rate is 4.096 Mbit/s than when it is 1.024 Mbit/s. If the chip rate is high, the chip time (time length per chip) will become short. On the other hand, if the range of fluctuation of traveling speed is large like vehicles, fluctuation of the Doppler shift will be large. If the communication speed is set at a high speed, fluctuation of the Doppler shift will be relatively large compared with the chip time. Therefore, when it was conventionally desired to perform the CDMA communication in a vehicle, there was no option but to restrict the communication speed.
The object of the present invention is to provide a radio communication system for mobile objects, such as a vehicle, whose traveling speed reaches high speed. In the system of the present invention, the CDMA communication can be satisfactorily carried out in the whole speed area, and the communication speed can be accelerated.
(1) In order to achieve the object mentioned above, a radio communication system for mobile objects according to the present invention is a system to carry out the CDMA radio communication between a mobile station and a base station, and comprises traveling speed detecting means for detecting a traveling speed of the mobile station, determining means for comparing the traveling speed with a prescribed speed determining value, and communication speed changing means for changing a communication speed between the base station and the mobile station when the traveling speed crosses the prescribed speed determining value.
Preferably, a radio communication system for mobile objects according to the present invention comprises a mobile station and a base station. The mobile station comprises traveling speed detecting means for detecting a traveling speed of the mobile station, determining means for comparing the traveling speed and a prescribed speed determining value, requesting means for requesting the base station to change a communication speed when the traveling speed crosses the prescribed speed determining value, and mobile station communication speed changing means for changing a communication speed on the mobile station side. The base station has base station communication speed changing means for changing a communication speed on the base station side at the request of the mobile station. The mobile station communication speed changing means of the mobile station changes a communication speed on the mobile station side adjusting to the change of the communication speed on the base station side.
According to the present invention, when the traveling speed is low, the communication speed is set at a high speed. When the traveling speed exceeds a predetermined reference speed value, the communication speed is changed to low. Thus, even though the traveling speed of the mobile object becomes high due to acceleration while performing high speed communication, the quality of communication will not be deteriorated. On the contrary, if the traveling speed becomes lower than a predetermined speed value due to deceleration of the mobile object, the communication speed will be changed to high. Therefore, the communication speed is accelerated according to deceleration of the mobile object, whereby data communication can be completed in a short time. Thus, according to the present invention, it is possible to perform satisfactory communication in the whole speed range of the mobile object, and it is possible to perform the communication as speedily as possible by setting a suitable communication speed in each traveling speed area. Suitable communication can be performed regardless of the traveling speed of the mobile object. Thus, in the CDMA communication, it is possible to make full use of the advantage of mobile communication that data communication can be performed in an arbitrary area
The present invention is suitable for the CDMA communication to which the direct sequence (DS) type spectrum diffusion described above is applied, as well as communication to which other types of spectrum diffusion are applied. These are, for example, frequency hopping (FH), time hopping (TH), or hybrid (DS/FH).
Further, the number of predetermined speed values and the number of steps to change the communication speed may be singular or plural. If a large number of predetermined speed values are set and the number of steps to change the communication speed is increased, the communication speed can be successively changed.
(2) Preferably, in the system according to the present invention, when the communication speed is changed, the chip rate of PN code or the bandwidth of a communication signal is changed.
If the chip rate of PN code is changed, the communication speed will change accordingly. If the chip rate is high, the communication speed will be also high. Further, if the chip rate is high, there will be a harmful influence of the Doppler shift while traveling at a high speed. According to the present invention, when the traveling speed is low, high speed communication will be carried out by increasing chip rate. When the traveling speed becomes high, the chip rate is lowered. Thus, the communication is satisfactorily performed while traveling at a high speed. The bandwidth of communication signal corresponds to the chip rate, so that if the chip rate is changed, the bandwidth will also be changed. Therefore, in changing the communication speed, the bandwidth may also be changed.
It is preferable to realize a change of the chip rate by deforming the PN code, as well as changing the PN code itself. In the latter case, the PN code to be used differs depending on whether the traveling speed is low or high.
(3) Preferably, in the system according to the present invention, communication can be performed between a mobile station and base station simultaneously using a plurality of PN codes, and the number of the PN codes simultaneously used is changed in changing the communication speed.
This aspect relates to a constitution in which the CDMA communication speed is increased more than it used to be. In this aspect, a plurality of PN codes are simultaneously used between a mobile station and a base station. Thus, compared with the case where a single PN code is used, the communication speed becomes high in proportion to the number of PN codes used. However, it is difficult to simultaneously process a plurality of PN codes unless the traveling speed of the mobile object is low. It is considered that fading is influential on this difficulty. As is well known, fading is a phenomenon in which an amplitude and a phase of an input signal fluctuate due to the influence of buildings or the like and it mainly results from multipaths. Instantaneous interruption (burst noise) at the time of traveling in the shade of an obstacle, such as a building, is also a cause of fading. Due to the influence of such fading, the number of PN codes enabling stable communication process will be limited if the traveling speed is high.
Therefore, when a plurality of PN codes are used, it is desired to solve the problem mentioned above. In other words, if it is assumed that the mobile object is at a standstill or travels at a low speed, a large number of PN codes can be set and high speed communication is possible. However, in order to apply to high-speed mobile objects, a small number of PN codes have to be set. Thus, the communication speed cannot be high.
However, according to the present invention, when the mobile object travels at a low speed, high speed communication can be performed as a relatively large number of PN codes are simultaneously processed. If the traveling speed becomes high, the number of PN codes will be reduced. Thus, satisfactory communication can be performed without being influenced by fading. On the contrary, if the traveling speed becomes low, high speed communication can be performed by increasing the number of PN codes. Thus, according to the present invention, satisfactory communication can be performed in the whole speed area, and it is possible to perform communication as speedily as possible by setting an appropriate number of PN codes in each traveling speed area.
The aforementioned aspect (3) may be applied to communication systems together with the constitution (2). It is also preferable to apply only the aspect (3) without applying the constitution (2).
(4) Preferably, in the communication system according to the present invention, a degree of spectrum diffusion is changed in changing the communication speed. In spectrum diffusion, a signal primarily modulated is further secondarily modulated using the DS system, FH system or the like described above, whereby the bandwidth is widened. The degree of spectrum diffusion is a ratio of the bandwidth after the secondary modulation to the bandwidth before the secondary modulation in the spectrum diffusion. When the degree of diffusion is changed, the communication speed is changed. Thus, an effect of the present invention is obtained. For example, as explained in the aforementioned aspect (2), a change of the degree of diffusion can be realized by changing the chip rate of PN codes. Besides, the degree of diffusion can be varied by changing the process in the secondary modulation.